The present invention relates generally to the measurement of inductance and eddy current losses. More particularly, the invention concerns a sensor that utilizes an inductive divider circuit to monitor the proximity of a target object.
Proximity sensors and switches incorporating the same are well known in the art. Such switches have proven to be particularly useful and reliable under the extreme environmental conditions encountered in aerospace applications. Exemplary applications including monitoring the positions of such items as landing gears, doors, slats, and thrust reversers. For such applications, the proximity switches have taken one of several forms. In the so-called two-piece proximity switch, the sensor unit includes an inductive sensor unit that is located at a remote site and interconnected by a cable to its associated electronic circuitry. In the so-called one-piece proximity switch, the sensor unit and the electronic circuitry are both contained within a single case or container. In such switches, the sensor unit is typically configured by positioning a coil within one-half of a standard ferrite pot core. As is well known in the art, the inductance of the sensor indicator and/or the eddy current losses varies in accordance with variations of the target-to-sensor distance. By determining the inductance of the sensor, or the eddy current losses therein, the distance relationship between the target and the sensor can be ascertained.
Efforts have been made in the prior art to make the proximity switch and the sensor unit physically robust in order to be resistive to the extremes of temperature, vibration, and shock, as well as the chemicals, corrosive fluids, and adverse moisture conditions encountered in operation. Despite these efforts, physical enviromental conditions, as well as other conditions such as electrical transients and electromagnetic interference (EMI), and still affect the accuracy of the inductance or eddy current loss measurement, and, consequently, the reliability of the switching of the proximity-sensing unit. The physical environmental effects, generally temperature variations, cause the parameters of the sensing coil as well as temperature-affected voltages and currents in the sensitive electronics to adversely affect the accuracy. The majority of the variation in the sensor is a result of the temperature coefficient of the wire, which affects both the AC and the DC losses in the sensor assembly. Permeability changes in the core and target, along with loss changes in the core, case, and target from temperature, also cause inaccuracies. In the electronic circuitry that interfaces with the sensor, these variations typically cannot be discriminated against and, thus, appear as apparent target "losses" that result in actuation variations.
In order to reduce the temperature dependencies of inductor proximity-sensing systems, prior art devices have incorporated temperature-compensating measures. For example, U.S. Pat. No. 3,454,869 discloses a temperature-compensated sensor unit for a two-piece proximity switch. Yet another temperature compensation technique for a two-piece proximity-sensing switch is dislosed in U.S. Pat. No. 4,219,740. In the arrangement shown in this patent, a time-varying current is established through the sensor unit. When the voltage across the sensor unit reaches a predetermined reference level, a sensor control loop controls the current through the sensor unit so as to constrain the sensor voltage constants at the reference level. A retrieval stage then determines the rate of change of the sensor current when the sensor current goes to zero and provides a voltage representative of the inductance of the sensor inductor. The action of the sensor control loop cancels the effect of the equivalent shunt capacitance of the sensor unit while the action of the retrieval network cancels the effect of the equivalent series resistance of the sensor unit. Since each of these parameters is variable with changes in temperature, the overall effect is to provide a temperature-independent measure of the inductance of the sensor inductor. While such arrangements do provide some degree of temperature compensation, they do so at higher power levels and with less stability than is obtainable with this invention.
Aside from temperature and other physical environmental effects, electrical transients and EMI are the principal cause of inaccurate detection of proximity. Such electrical noise can enter the unit, either through the sensor unit or body or through the cable or wiring that connects the sensor unit to the electronic circuity or that interfaces the switch with other electronic circuitry.